The invention relates to communication systems generally and, more particularly, to communication systems and methods for communicating with positioning and sensing devices connected to underwater hydrophone cables.
Conventional hydrophone cables, such as streamers towed behind survey vessels, are equipped with positioning and sensing devices, such as cable-leveling or -steering birds, acoustic transceivers, and heading sensors. The devices may be mounted external to the streamer or in-line with it. An unshielded twisted-pair line runs the length of each streamer. Communication coils connected at spaced apart locations along the length of the twisted-pair line inductively couple commands, control, and data signals between the communication line and the devices. A controller aboard the survey vessel towing the streamers transmits messages to and receives data from the devices over the twisted-pair line. In conventional systems, the twisted-pair communications operates at 2400 baud (with 1 bit per symbol) half-duplex with frequency-shift-keying (FSK) modulation at a center frequency of 26 kHz.
As longer and longer streamers with more and more devices are placed into service, the 2400-baud communication system becomes a factor limiting further expansion. First, the signals attenuate along the length of the streamer and, consequently, limit the useful length of the streamer. Repeaters that boost the signals along the length of the streamer have been deployed to solve the problem of signal attenuation. But, even with repeaters, the increased number of devices means more time required for communications and, at 2400 baud, the communications bandwidth is limited.
Thus, there is a need for a communication system that supports longer streamers and more devices on existing unshielded, twisted-pair communication lines.